Human cytomegalovirus (HCMV) is a betaherpesvirus which causes clinically serious disease in immunocompromised and immunosuppressed adults, as well as in some infants infected in utero or perinatally (Alford, C. A., and W. J. Britt. 1990. Cytomegalovirus, p. 1981-2010. In D. M. Knipe and B. N. Fields (ed.), Virology, 2nd ed. Raven press, New York). In human cytomegalovirus (HCMV)-infected cells, expression of the cellular major histocompatibility complex (MHC) class I heavy chains is down-regulated. The 230-kb dsDNA genome of HCMV was sequenced (Chee, M. S., A. T. Bankier, S. Beck, R. Bohni, C. M. Brown, R. Cerny, T. Horsnell, C. A. Hutchinson, T. Kouzarides, J. A. Martignetti, E. Preddie, S. C. Satchwell, P. Tomlinson, K. Weston, and B. G. Barrell. 1990. Analysis of the protein-coding content of the sequence of human cytomegalovirus strain AD 1 69. Curr. Top. Microbiol. Immunol. 154:125-169) and has at least 200 open reading frames (ORFs). The functions of most of these 200 genes is unknown. The function of some HCMV proteins are known or predicted due to their homology with other viral (especially herpes simplex virus) and cellular proteins. However, for the majority of the HCMV ORFS, the functions of the proteins they encode is unknown.
Several investigators have shown that infection by HCMV results in the down-regulation of cellular MHC class I heavy chains (Browne, H., M. Churcher, and T. Minson. 1992. Construction and characterization of a human cytomegalovirus mutant with the UL18 (class I homolog) gene deleted. J. Virol. 66:6784-6787; Beersma, M. F. C., M. J. E. Bijlmakers, and H. L. Ploegh. 1993. Human cytomegalovirus down-regulates HLA class I expression by reducing the stability of class I H chains. J. Immunol. 151:4455-4464; Yamashita, Y., K. Shimokata, S- Mizuno, H. Yamaguchi, and Y. Nishiyama. 1993. Down-regulation of the surface expression of class I MHC antigens by human cytomegalovirus. Virology 193:727-736). Down-regulation is defined as reduction in either synthesis, stability or surface expression of MHC class I heavy chains. Such a phenomenon has been reported for some other DNA viruses, including adenovirus, murine cytomegalovirus, and herpes simplex virus (Anderson, M., S. Paabo, T. Nilsson, and P. A. Peterson. 1985. Impaired intracellular transport of class I MHC antigens as a possible means for adenoviruses to evade immune surveillance. Cell 43:215-222; Burgert and Kvist, 1985, Cell, 41:987-997; del Val, M., K. Munch, M. Reddehasse, and U. Koszinowski. 1989. Presentation of CMV immediate-early antigen to cytotoxic T lymphocytes is selectively prevented by viral genes expressed in the early phase. Cell 58:305-315; Campbell, A. E., J. S. Slater, V. J. Cavanaugh, and R. M. Stenberg. 1992. An early event in murine cytomegalovirus replication inhibits presentation of cellular antigens to cytotoxic T lymphocytes. J. Virol. 66:3011-3017; Campbell, A. E., J. S. Slater. 1994. Down-regulation of major histocompatibiiity complex class I synthesis by murine cytomegalovirus early gene expression. J. Virol. 68:1805-1811; York, I. A., C. Roop, D. W. Andrews, S. R. Riddell, F. L. Graham, and D. C. Johnson. 1994. A cytosolic herpes simplex virus protein inhibits antigen presentation to CD8+ T lymphocytes. Cell 77:525-535). In the adenovirus and herpes simplex virus systems, the product of a viral gene which is dispensable for replication in vitro is sufficient to cause down-regulation of MHC class I heavy chains (Anderson, M., S. Paabo, T. Nilsson, and P. A. Peterson. 1985. Impaired intracellular transport of class I MHC antigens as a possible means for adenoviruses to evade immune surveillance. Cell 43:215-222; Burgert and Kvist, 1985, supra). The gene(s) involved in class I heavy chain down-regulation by murine cytomegalovirus have not yet been identified.
HCMV causes benign but persistent infections in immunocompetent individuals implying a balance between immune control of and immune escape by the virus (Rinaldo, C. R. (1990) Annu. Rev. Med. 41, 331-338). Cytotoxic T cells play a major role in the immune defense against most viruses, since infected cells can be lysed upon engagement of T-cell receptors with MHC class I molecules presenting virus derived peptides (Townsend, A. & Bodmer, H. (1989) Ann. Rev. Iiiiitiui ol. 7, 601-624). However, HCMV infection prevents T cells from recognizing viral or self antigens if they are synthesized within the cell, but not if the specific epitope is given as a peptide (Hengel, H., Esslin-er, C., Pool, J., Goulmy, E. & Koszlnowski, U. H. (1995) J.GenVirol 76, 2987-2997), consistent with HCMV interference with MHC class I assembly and a peptide transport. In non-infected cells, newly synthesized MHC class I heavy chains (HC) associate with .beta.2-microglobulin (.beta.2m) in the endoplasmic reticulum. Subsequently, heterodimers associate with TAP, an ATP-driven peptide transporter, and acquire 8-10 amino-acid long peptides imported from the cytosol (Heemels, M.-T. & Ploegh, H. (1995) Ann. Rev. Biocliem. 64, 463-491). This trimeric complex is then transported to the cell surface provided peptides are bound with sufficient affinity. By contrast, HCs are selectively degraded immediately after synthesis in HCMV-Infected cells (Warren, A. P., Ducroq, D. H., Lehner, P. J. & Borysiewicz, L. K. (1994) J Virol 68, 1822-2829; Yamashita, Y., Shimokata, K., Saga, S., Mizuno, S., Tsurumi, T. & Nishiyama, Y. (1994) J Virol 68, 7933-7943; Beersma, M. F. C., Bijlmakers, M. J. E. & Ploegh, H. (1993) J Iinm tiol. 151, 4455-4464). Deletion analysis has revealed that at least two loci within the growth-dispensable US-region of the HCMV genome mediate such degradation independently (Jones, T. R., Hanson, L. K., Sun, L., Slater, J. S., Stenberg, R. M. & Campbell, A. E. (1995) Journal of Virology 69, 4830-41). One of the responsible genes was identified as US 11, since US11 transfectants faithfully reproduced viral HC degradation. Interestingly, HCs appear to be exported into the cytosol for degradation in US11 transfectants (Wiertz, E. J. H. J., Jones, T. R., Sun, L., Bogyo, M., Geuze, H. J. & Ploegh, H. L. (1996) Cell 84, 769-779).